In order to determine the distribution or effect of fluids in oil-bearing rock formations from test samples it is common to saturate a core sample of the rock with one of the fluids and to submerge the sample in another fluid in a centrifuge chamber. Typically, one of the fluids is oil from the rock formation and the other is water or gas. Because the fluids are immiscible and of different densities, when the centrifuge is rotated some of the fluid in the sample is expelled and replaced by the other fluid. This can be noted and measured through a window provided in the bucket.
When dealing with an oil-bearing formation that is impacted by both gas and water displacement, such as studies involving the effects of water injection in a gas gravity drainage field, it has been necessary to use two different centrifuge buckets in two separate experiments conducted at ambient conditions. Typically, an oil-saturated core sample is placed in a centrifuge bucket and gas is injected into the centrifuge chamber. When the centrifuge is rotated gas will expel some of the oil in the sample. Because the oil has a greater specific gravity than the gas, it collects in the window area at the outer extremity of the bucket. Then the oil-saturated sample, which now contains only the oil which was not expelled during the above experiment, is submerged in a water-filled centrifuge bucket of different design wherein the window area is at the inner extremity of the bucket. Upon rotation of the centrifuge oil is expelled from the sample and, having a lower specific gravity than water, collects in the window area at the inner extremity of the bucket. The results of both experiments then have to be integrated in order to determine the calculated effect of the gas and water in the formation. This is not an entirely reliable procedure, however, because the experiments are not conducted under reservoir conditions, and the actual conditions in the reservoir are often not adequately reflected in a model based on the data collected. This problem is made worse by the fact that the sample loses its gas saturation from the first experiment, thereby not enabling actual reservoir conditions to be reflected during the second experiment.
Another approach to the study of a three phase gas/water/oil system is to use coreflooding experiments. Such procedures require the use of large cores, in the order of four to five feet long, which have to be flooded at an extremely slow rate, often requiring months to complete the operation.. Moreover, capillary pressure causes oil to be held in the ends of the core. Since such oil would not normally be present in a reservoir, this tends to give misleading results. Coreflooding experiments are therefore not practical as a means of quickly obtaining an accurate model of such a reservoir. Coreflooding experiments are, in addition, quite expensive to carry out.
It would be desirable to be able to obtain data which more realistically reflects actual conditions in a reservoir affected by both gas and water pressure, and to do so rapidly, accurately and economically. Further, it would be desirable to be able to collect such data in a single experiment rather than in two separate experiments.